container/heap: optimization in case heap has many duplicates

benchmark       old ns/op    new ns/op    delta
BenchmarkDup      3075682       609448  -80.18%

R=gri
CC=golang-dev
https://golang.org/cl/6613064

src/pkg/container/heap/heap.go

@@ -79,7 +79,7 @@ func Remove(h Interface, i int) interface{} {
 func up(h Interface, j int) {
 	for {
 		i := (j - 1) / 2 // parent
-		if i == j || h.Less(i, j) {
+		if i == j || !h.Less(j, i) {
 			break
 		}
 		h.Swap(i, j)
@@ -97,7 +97,7 @@ func down(h Interface, i, n int) {
 		if j2 := j1 + 1; j2 < n && !h.Less(j1, j2) {
 			j = j2 // = 2*i + 2  // right child
 		}
-		if h.Less(i, j) {
+		if !h.Less(j, i) {
 			break
 		}
 		h.Swap(i, j)

src/pkg/container/heap/heap_test.go

@@ -170,3 +170,16 @@ func TestRemove2(t *testing.T) {
 		}
 	}
 }
+
+func BenchmarkDup(b *testing.B) {
+	const n = 10000
+	h := make(myHeap, n)
+	for i := 0; i < b.N; i++ {
+		for j := 0; j < n; j++ {
+			Push(&h, 0) // all elements are the same
+		}
+		for h.Len() > 0 {
+			Pop(&h)
+		}
+	}
+}